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Why T-rex had tiny forelimbs

quaShort answer:T-rex(Figs. 1, 2) forelimbs were former wings, not former grasping, predatory tools. Kiwi wings (Fig. 3), which have claw tips, are good analogs. Tyrannosaurus forelimbs were relatively smaller and likewise useless. Taxon exclusion is once again the reason why this has not been able to be documented before.

What others say:Science Daily – “The tiny arms on the otherwise mighty Tyrannosaurus rex are one of the biggest and most enduring mysteries in paleontology.”

Thoughco.com – “T. Rex males mainly used their arms and hands to grab onto females during mating (females still possessed these limbs, of course, presumably using them for the other purposes listed below). T. Rex used its arms to lever itself off the ground if it happened to be knocked off its feet during battle, T. Rex used its arms to clutch tightly onto squirming prey before it delivered a killer bite with its jaws. they were exactly as big as they needed to be. This fearsome dinosaur would quickly have gone extinct if it didn’t have any arms at all.”

Popularmechanics.com – “The simple truth is that scientists aren’t sure exactly why T. rex’s arms are so short, but there’s a number of possible explanations. Perhaps the most likely is that the dino’s arms just weren’t very useful.”

FieldMuseum.org – “One of the big mysteries about T. rex is its tiny forelimbs,” says Pete Makovicky, Associate Curator of Dinosaurs. “We don’t know how it used them. But there could be clues in the fossils. When a bone is used a lot, the wear and tear cause tiny fractures that heal over time. With the right tools, we can see microscopic changes in the bones caused by that healing process. You also see things like a wider bone marrow cavity. When we remove SUE’s arm, we’re going to take it to the Argonne National Laboratory to try to look for these characteristics that will tell us how much it was used.”

The answer, as usual here, comes from phylogenetic analysis.
Distinct from prior tyrannosaur studies, the large reptile tree (LRT, 1040 taxa) recovers the feathered, winged theropod, Zhenyuanlong (Fig. 1). as the proximal ancestor to the tyrannosaur clade. Theropods with large feathered wings don’t use them to grasp prey or mates.

Figure 1. Distinct from prior studies, Ornitholestes, Tianyuraptor and Zhenyuanlong are close relatives of Tyrannosaurus rex in the large reptile tree. Here Zhenyuanlong preserves long feathers on its forelimbs.

No other studiesfound large feather wings on tyrannosaur ancestors. And as long as they don’t, they’ll keep thinking T-rex forelimbs were primitive grasping organs.

Figure 2. Taxa in the Compsognathus/Tyrannosaurus clade, a subset of the large reptile tree to scale. Also included are Microraptor, Sinornithosaurus, Dilong and Zhenyuanlong.

The kiwi forelimb is a good T-rex forelimb analogThe kiwi (Fig. 2) has vestigial forelimbs that are essentially useless. Even so, they were relatively much large than T-rex forelimbs. Kiwis have no trouble getting up, mating or anything else tyrannosaurs are supposed to do with their forelimbs.

Figure 3. Kiwi skeleton GIF animation (2 frames) showing the vestigial and useless forelimb tipped with a claw, an analog to the vestigial forelimb of T-rex.

A recent lectureby Tyrannosaur Chronicles author Dr. David Hone, available here on YouTube, noted two traits common to all tyrannosaurs: fused nasals and D-shaped (in cross-section) premaxillary teeth. Zhenyuanlong does not have these traits. Thus the Hone traits have not been validated by the LRT. Instead those traits appear to have a wider distribution by convergence, like the arctometatarsals we looked at earlier.

Figure 6. Tyrannosaurus forelimb compared to Gorgosaurus. Note the larger coracoid in T-rex. It might have been resting on it.

Remember
we never want to put all our trust in just one or two traits (see above). Otherwise we’d be pulling a Larry Martin, famous for arguing phylogeny based on one or two traits alone. Instead we’re always looking for a suite of traits based on a character list of at least 150. The LRT has 228 multi-state characters and it continues to lump and separate all of its 1040 included taxa successfully, while documenting gradual accumulations of derived states.

In the present ancestry of tyrannosaurs other traits emerge.Among the 228 traits, the LRT found several dozen shared by T-rex and Zhenyuanlong, including the elevated orbit, the hourglass-shaped quadratojugal, the pubic boot and a very short dorsal vertebral series. Noteworthy, all of these traits, other than the hourglass-shaped quadratojugal are also found elsewhere in the LRT by convergence. Don’t forget, we’re looking for the most parsimony in a suite of traits. Otherwise Pinnipedia and Cetacea would still be valid clades.

Essentially,T-rex is just a giant, flightless Zhenyuanlong. No longer small enough to fly, the feathered flapping organs of T-rex became smaller due to lack of use. Blame it on the genes that those useless forelimbs keep appearing. We’ve also seen vestigial traits in pterosaurs (manual digit 5, ungual 4, pedal digit 5 in derived taxa), snake precursors (legs) and in baleen whales (tooth buds in embryos).

Postscript
Within 24 hours of this post T. Kaye alerted me to Giffin 1995 who wrote: “the data suggest that the brachial plexus, and therefore the cervical/dorsal vertebral transition, of the theropod dinosaurs studied was located considerably posterior to its presently accepted location, and that the forelimbs of the giant carnosaurs Tyrannosaurus rex and Carnotaurus sastrei were of biologically insignificant use.”

4 thoughts on “Why T-rex had tiny forelimbs”

Very interesting article. Now, please forgive me if I have got hold of the wrong end of the stick as I am not a professional. You are saying the tyranosaurs are secondarily flightless, and I have read that oviraptors may also be secondarily flightless, this from “Avian Evolution” by Gerald Mayr, though he only mentions it as a possibility, not anything more solid. So, based on the possibility of both these being secondarily flightless, can it then be seen that some basal coelurosaurs may have become at least gliders before they went their different ways to become tyranosaurs and the many types of maniraptor, so putting the origins of dinosaur flight back much further than previously thought. Or am I putting 2+2 together and making 5.

Yes, I had grasped the wrong end of the stick somewhat. I see the word “wings” and it still conjures a view of something to do with flying.

If you don’t mind, I have a few more questions, which I hope are not wildly off track.

I certainly agree that T-rex arms were not used for pushing itself off the ground or any form of grasping. I also do not go with the idea that as it’s arms seem to be reasonably powerful, I think some have claimed they may have been as strong as a strong mans, that it engaged in any form of lifting. That idea seems ridiculous. So I presume for display of some kind in the earliest tyranosaurs, and the limbs gradually decreasing in size as their body size, particularly head, increased, weight saving?

So presuming the limbs were used for some kind of display, what type of feather would have covered them. Is there the possibility of pennaceous feathers appearing on tyranosaur forelimbs before the emergence of maniraptora, or alongside them, or from an as yet undiscovered basal coelurosaur that had a bit more than just “dinofuzz”.

I’m going to stick with my original proposal… like the kiwi they were essentially useless, but genes still produced them. Covering materials would have to be completely imagined. It could go with furry filaments or tiny pennaceous feathers.